Conventionally, when manufacturing a spectacle lens from a circular lens base having an unfinished convex surface, it is fabricated by cutting or grinding the convex surface of the lens base into a predetermined surface shape by a numerically controlled grinding machine, e.g., a versatile polishing apparatus TORO-X2SL manufactured by LOH, to have a thickness slightly larger than the finishing size including the lapping stock or polishing stock, and then polishing the convex surface into a predetermined curved surface by the polishing apparatus.
According to U.S. Pat. No. 5,421,770, in cutting or polishing a lens base, the lens base is attached to a polishing apparatus through a lens holding tool which is bonded to a non-polishing surface.
According to the present invention, in the following description, the lens base will be referred to as a lens blank, and the lens holding tool will be referred to as a lens holding unit. To fix the lens blank to the lens holding unit through a bonding agent will be referred to as blocking or block. A device that performs such blocking or block will be referred to as a blocking device. As the blocking device, for example, a device called a layout blocker manufactured by LOH is known.
Concerning blocking a lens blank, in the blocking device disclosed in Japanese Patent Laid-Open No. 2003-334748, as shown in FIG. 13, a lens blank 1 is disposed above a lens holding unit 2 through a blocking ring 3. A molten bonding agent 4 is poured into the space surrounded by three members, i.e., the lens blank 1, lens holding unit 2, and blocking ring 3, and cooled to solidify so as to block the lens blank 1 by the lens holding unit 2. As the bonding agent, generally, an alloy or wax having a low melting point is employed.
In this blocking device, different types of lens holding units 2 and blocking rings 3 are prepared to correspond to the types of the lens blanks 1. In blocking, a lens holding unit 2 and blocking ring 3 corresponding to the employed lens blank 1 are selected and used so the center of the bonding agent 4 has a predetermined thickness. In FIG. 13, reference numeral 5 denotes a table; and 6, a fixing means for fixing the blocking ring 3 to the table 5.
When the lens blank 1 is to be blocked by the lens holding unit 2, the center of the lens blank 1 must accurately coincide with the center of the lens holding unit 2. For this purpose, in the centering devices disclosed in Japanese Patent Laid-Open No. 09-290340, 11-325828, and the like, the lens blank 1 is clamped and centered with respect to the lens holding unit 2.
The centering device described in Japanese Patent Laid-Open No. 09-290340 is a device that mechanically centers a lens with reference to its peripheral surface. During blocking, a ring member surrounding the lens is pivoted to urge the holding portions of three lever members against the peripheral surface of the lens so as to center the lens.
Japanese Patent Laid-Open No. 11-325828 relates to a measuring method and apparatus with which, to an optical component having a concave surface, e.g., a concave lens or concave mirror, the center position of the concave surface and the center position of the outer shape of the optical component are obtained. According to this apparatus, the edge portion of the concave surface of the optical surface is cut to form a flat surface. The coordinates of at least three points on a circle surrounded by the flat surface are measured by a differential interference microscope and a distance measuring device which measures the moving amount of a moving table. The center position of the circle is calculated from the measured coordinates, and the calculated center position is determined as the center position of the concave surface.